Thermosensitive recording media are ordinarily prepared by mixing together a colorless or pale-colored electron donating leuco dye and an electron accepting color developing agent, such as a phenolic compound and the like, after grinding them into fine particles, preparing a coating solution by adding a binder, a filler, a sensitivity enhancing agent, a slipping agent and other aids to the mixture and applying the coating solution onto a substrate such as paper, synthetic paper, film, plastic and the like. Thermosensitive recording medium develops color through an instantaneous chemical reaction when heated using a thermal head, hot stamp, hot pen, laser light and the like and yield a recorded image. Thermosensitive recording media are used extensively in recording media such as facsimile devices, computer terminal printers, automatic ticket dispensers, recorders for meters, receipts at super markets and convenience stores and the like. Increasing numbers of receipt recording media print advertisements on the receipt back side, and overall printability (print through resistance, adhesion, printing work properties and the like) is sought in addition to the previously demanded qualities such as color development sensitivity, image quality and the like.
When the back side of a thermosensitive recording medium is subjected to general purpose printing, the ink penetrates to the other side (the surface on which a thermosensitive recording layer is present) and creates a problem (that is, a print through problem) associated with the difficulty reading the letters printed on the thermosensitive recording layer and the like when the degree of substrate opacity is inadequate in the thermosensitive recording medium. Therefore, improving the opacity of the substrate in a thermosensitive recording medium becomes important.
Increasing bulk is commonly used as a method to improve the degree of opacity in paper. In the area of thermosensitive recording medium, a thermosensitive recording medium with improved color developing sensitivity obtained by using a substrate to which a bulking agent such as polyvalent alcohol and the like is added was disclosed. (Reference 1)
In addition, mechanical pulp is used as the material for newspaper, magazines and the like, and the use of mechanical pulp is commonly known to increase the bulk in paper. (Reference 2 etc.)
In addition, a method to suppress the influence that ink jet ink has on a thermosensitive recording surface in a thermosensitive recording medium with ink jet recordability on the back side by installing two undercoating layers between the substrate and the thermosensitive recording layer was disclosed. (Reference 3)